1. Field of the Invention
The present invention relates to a dry etching method employed in such applications as production of semiconductor devices. More particularly, it relates to a dry etching method whereby a silicon oxide based material layer and a silicon nitride based material layer may be etched with high selectivity for each other.
2. Description of the Prior Art
The recent trend toward higher integration and performance of such semiconductor devices as VLSIs and ULSIs requires dry etching technologies for insulation films to achieve correspondingly higher anisotropy, higher etchrate, higher selectivity, lower pollution, and less damage with no compromise in these requirements.
Conventionally, etching gases typified by CHF.sub.3 gas, CF.sub.4 /H.sub.2 mixed gas, CF.sub.4 /O.sub.2 mixed gas, and C.sub.2 F.sub.6 /CHF.sub.3 mixed gas have been widely used to etch an insulation film composed of silicon oxide (SiO.sub.x ; particularly, x=2). All these etching gases are composed mainly of fluorocarbon based gas whose molecule has C/F ratio (the ratio of the numbers of carbon atoms to that of fluorine atoms in one molecule) of 0.25 or higher. The common functions of these gases include: (a) forming a C--O bond from a constituent element C on the surface of a SiO.sub.2 layer and cleaving or weakening an Si--O bond, (b) forming CF.sub.n.sup.+ (particularly, n=3) as a main etchant for an SiO.sub.2 layer, and (c) generating relatively carbon-rich plasma and thereby removing oxygen from SiO.sub.2 in the form of CO or CO.sub.2 while achieving a lower etchrate and higher selectivity for an underlying silicon layer as C, H, F, and other constituent elements contribute to deposition of carbonaceous polymers on the surface of the underlying layer.
It is to be noted that the above mentioned H.sub.2 and O.sub.2 are used to regulate selectivity for an underlying silicon layer, in other words, the apparent C/F ratio of an etching reaction system by increasing or decreasing the quantity of F*.
Basically, etching gases for an SiO.sub.2 layer are also used to etch an insulation layer composed of silicon nitride (Si.sub.x N.sub.y particularly, x=3 and y=4). While the SiO.sub.2 layer is etched mainly through an ion-assisted reaction, the Si.sub.x N.sub.y layer is etched at a higher rate through a radical reaction using F* as a main etchant. Such an etchrate difference is somewhat predictable from the descending order in binding energy of an Si--F bond (132 kcal/mole), Si--O bond (111 kcal/mole), and Si--N bond (105 kcal/mole). Incidentally, these binding energy values are cited from data shown in "Handbook of Chemistry and Physics" 69th Edition (1988) edited by R. C. Weast (published by CRC Press Inc. in Florida, U.S.A.) and may vary slightly according to any other calculation method.
Meanwhile, the recent trend toward higher integration of semiconductor devices requires correspondingly higher selectivity for an SiO.sub.x layer and an Si.sub.x N.sub.y layer.
For instance, an Si.sub.x N.sub.y layer formed on an SiO.sub.x layer as an underlying layer is etched in the LOCOS method where the Si.sub.x N.sub.y layer is patterned to define an element isolation region. This etching process requires extremely high selectivity for the underlying layer now that a pad oxide film (SiO.sub.2 layer) is decreased in thickness to minimize bird's beak length.
On the other hand, an SiO.sub.x layer formed on an SiN.sub.x layer as an underlying layer must be etched, for instance, in a contact hole forming process. In recent years, a thin Si.sub.x N.sub.y layer may be formed between the SiO.sub.x interlayer insulating film and the Si substrate for the purpose of reducing damage to the substrate in an over-etching process. To achieve this purpose, this etching process also requires extremely high selectivity for the underlying SiN.sub.x layer.
In principle, when achieving high selectivity for two laminated dissimilar material layers, it is preferable that there should be a great difference in interatomic bond energy between the chemical bonds of the layers. In the case of an SiO.sub.x layer and an Si.sub.x N.sub.y layer, in particular, it is essentially difficult to achieve high selectivity between these two layers because of little difference in bond energy between the bonds thereof (Si--O bond and Si--N bond, respectively) and common bases of the etching gas thereof. Therefore, persistent efforts have been made in industrial sectors to develop technologies for overcoming such difficulty.
In fact, some technologies have been reported for etching an Si.sub.x N.sub.y layer formed on an SiO.sub.x layer with high selectivity between these two layers.
For instance, the present inventor has disclosed in Japanese Patent "KOKAI" 61-142744 (1986) one such technology whereby CH.sub.2 F.sub.2 or any other gas with a low C/F ratio are used as etching gas with CO.sub.2 added at a molar ratio of 30 to 70%. More specifically, such gases with a low C/F ratio will form CF.sub.x.sup.+ (particularly, x=3) as an etchant for an SiO.sub.x layer only by recombining with F*. The quantity of CF.sub.x.sup.+ thus formed will decrease when a great quantity of CO* is fed to capture F* for removal from an etching reaction system in the form of COF. As a result, an etchrate for the SiO.sub.x layer will also decrease. Meanwhile, an Si.sub.x N.sub.y layer is etched by other ions than CF.sub.x.sup.+ and radicals, so that the etchrate thereof will remain almost unchanged even when a great quantity of CO.sub.2 is fed to the etching reaction system. Thus, high selectivity will be achieved between the SiO.sub.x layer and the Si.sub.x N.sub.y layer.
"Proceedings of Symposium on Dry Process", Vol. 88, No. 7, p. 86-94 (1987) has also reported another technology whereby an Si.sub.x N.sub.y layer formed on an SiO.sub.x layer is etched by FCl which will be formed in the gaseous phase by microwave discharge when NF.sub.3 and Cl.sub.2 are fed to a chemical dry etching apparatus. This technology is based on the fact that an Si--N bond with a 30% ionicity has a stronger covalency than an Si--O bond with a 55% ionicity. Namely, the Si.sub.x N.sub.y layer, whose chemical bond (Si--N bond) is similar in nature to the chemical bond (covalent bond) of single-crystal silicon, will be etched by F*, Cl*, and other radicals resulting from dissociation of FCl while the SiO.sub.x layer will almost never be etched by these radicals. Thus, high selectivity will also be achieved between the two layers.
As mentioned above, some technologies have been reported for selectively etching an Si.sub.x N.sub.y layer formed on an SiO.sub.x layer. This is a natural consequence considering the difference in etchrate between the two layers. When the Si.sub.x N.sub.y layer is etched mainly through a radical reaction, exposure of the SiO.sub.x layer to a plasma in the etching process will inevitably result in reduction of the etchrate.
However, there are some difficulties with the conventional technologies. For instance, anisotropic etching is essentially difficult with the above-mentioned technology using FCl because it is based on a radical reaction.
Conversely, no technology has been reported for selectively etching an SiO.sub.x layer formed on an Si.sub.x N.sub.y layer. This etching process etches the SiO.sub.x layer mainly through an ion-assisted reaction and involves even greater difficulty in achieving high selectivity between the two layers because radicals formed invariably in the etching reaction system will increase the etchrate upon exposure of the Si.sub.x N.sub.y layer to the plasma. In fact, it is certain that this etching process will be needed in the future and it is required to realize it as soon as possible.